The user-mode port of the Linux kernel represents an interesting and potentially
significant addition to the kernel pool. It is the first virtual kernel running
on Linux, except possibly for VMWare. As such, it places new demands on the
host kernel, possibly resulting in new functionality, which may then be used
by other applications. This has already happened. Until 2.2.15 and 2.3.22, ptrace
on Linux/i386 was not able to modify system call numbers (and on other architectures,
it still can't). The demands of this port prompted Linus and Alan to add that
feature, at which point several other applications started using it. At least
one of those applications was completely impossible beforehand.

Aside from this port, the only options for running virtual machines are VMWare
and IBM VM. In both of those cases, potential applications maybe be ruled out
for economic reasons or because VM doesn't run on the application platform.
The availability of a free virtual machine running on Linux may open up new
opportunities that were reserved for mainframes or which just didn't exist before.